Using autopsy knee joint specimens, 'laxity' will be measured on the intact knees, after alterations to simulate mild and severe arthritis and then with different total knee prostheses inserted. The objects are to determine 'normal' laxity in rotation and anterior- posterior modes, and to determine the design features of total knee prostheses which restore the altered (arthritic) knee to as normal a condition as possible. Laxity is used as a criterion because of its importance in obtaining normal function of the knee, and because prostheses incorporating laxity should produce better function and be more durable. Laxity (e.g. rotary) will be measured by cyclically loading the joints in an automatic machine, and obtaining torque-rotation graphs. Static laxity, the rotation for a small torque, describes the looseness in the joint before significant soft tissue restraint. 'Dynamic laxity' measures the restraint provided by the soft tissues at the extremes of motion. The results should give valuable data about the roles of soft tissues in controlling knee stability, and indicate the best designs of knee prostheses to use in different cases.